Local playback should be fine...
* What's the proc in your computer?
* How many processes do you have running/resources being used? Take a look at your Task Manager (CTRL+ALT+DEL > Task Manager)
* What speed is your harddrive? 5400, 7200?
* How did it look when you analyzed your drive(s) with the defreg utility?
* What's your internet connection like? http://www.speedtest.net/
* What type of video file did you render (Flash, .mov, .wmv, avi, etc..)?
* Does this happen when you try to playback any other video locally on your computer?
* Do you have a running nose or fever?